There are small-scaled power systems called a microgrid or the like supplying power to consumers in a specific region by using plural kinds of distributed power supplies (see Patent document 1) such as an engine generator, a turbine generator, a power storing apparatus, a fuel cell, and a natural energy generating apparatus such as solar power generation and wind power generation which are provided in the specified region.
If a small-scaled power system is interconnected to a commercial power system of a power company, there are cases of a normal power flow in which power is received from the commercial power system and a reverse power flow in which power is supplied to the commercial power system.
In any case, power supply and demand control of a distributed power supply in a small-scaled system is required in which a power flow (electric energy amount) at any point in the power system, or a deviation between a load power and a power generation output in the small-scaled power system is kept constant.
In the power supply and demand control of the small-scaled power system, a secondary battery is used with an engine generator, a turbine generator and a fuel cell to absorb a power fluctuation generated in the small-scaled power system. Generally, the secondary battery absorbs a comparatively fast output fluctuation generated in a natural energy generating apparatus of the small-scaled power system and an abrupt load fluctuation generated in the small-scaled power system.    Patent Document 1: Japanese Patent Laid-Open No. 2001-327080